In a Long Term Evolution (hereinafter referred to as LTE) system, a Random Access Channel (hereinafter referred to as RACH) is mainly used for initial access of a User Equipment (UE), and it carries no user data. A signal sent by the UE on the RACH channel is a Preamble Sequence (hereinafter referred to as Preamble), and the preamble sequence is a Zadoff-Chu sequence (hereinafter referred to as ZC sequence). As shown in FIG. 1, the 3GPP TS 36.211 Specification stipulates that, the Preamble includes two parts: a Cyclic Prefix (hereinafter referred to as CP) with a length of TCP, and an access sequence (hereinafter referred to as sequence or SEQ) with a length of TSEQ.
Meanwhile, the Specification stipulates several parameter settings in different formats to match different cell radiuses, as shown in Table 1:
TABLE 1Preamble SequenceMaximum CellFormat NumberTCPTSEQRadius (km)0 3168 · Ts24576 · TsAbout 14.6121024 · Ts24576 · TsAbout 77.32 6240 · Ts2 · 24576 · TsAbout 29.6321024 · Ts2 · 24576 · TsAbout 1004 448 · Ts 4096 · TsAbout 1.4Note:Ts is a basic time unit in the LTE Specification and Ts = 1/(15000 × 2048)s.
The LTE system is optimized in a low-speed case from 0 to 15 km/h, still has relatively high performance at a higher moving speed (e.g., 15-120 km/h), and the LTE system could also maintain connection at 120 km/h to 350 km/h. Depending on different features of system carrier frequency, the highest speed of the UE allowed by a current standard can be up to 500 km/h (TS25.913-900).
However, with the development of communication technologies and the development of users' requirements for communication, carriers need to provide services to higher speed vehicles. For example, requirements are brought forward for LTE communication covering a flight mode. A moving speed of the UE is higher in a flight case, which could be up to 1200 km/h, and therefore, a Doppler frequency shift is greater if the carrier frequency is same. On the other hand, to reduce network deployment costs, a larger cell radius needs to be supported, as the cell radius can even exceed 200 km, and a large cell causes a Round Trip Delay (hereinafter referred to as RTD or round trip delay) to become larger. These conditions all bring forward challenges to a random access technology.